Stephen Dorril, an expert on MI6, answers questions on what might have happened, and what impact it could have.

How secure is the building?

It's pretty secure in that it's surrounded by security guards. There are a number of security staff, and to get into the building requires a pass and a swipe card. It would be difficult for an outsider to gain entry. I would be extremely surprised if a bomb had been taken in. If that was the case, however, I think heads would roll.

Having said that, it's not the most secure building in the world because it's so obvious, it faces onto the river, it's lit up at night. Everyone knows it, it's been in a James Bond film, and it wasn't designed for an intelligence agency. It was later adapted for MI6.

The most secure part of it though is in the basement, which houses the computer and control room... It's a very highly secure part of the building.

Are explosive materials kept there?

I would be very surprised if there was any kind of explosives there [which was involved], especially as the explosion was on the eighth floor. As I understand it, the research laboratories are at different places.

If much damage is done, would it be possible for them to move operations elsewhere?

It would be very difficult to transfer operations, although one assumes they have made arrangements for this kind of incident. But it seems to me that it's relatively minor damage to the building. I don't see a real difficulty in carrying on with operations.

The communications might have been hit, but there should be back-up arrangements made. Having said that, though, you can never tell with British intelligence. They have made a number of balls-ups in the past, such as the loss of laptop computers.

What potential enemies does the organisation have?

It's very difficult to think of groups currently operational that are capable of carrying out a terrorist campaign of this sort in central London. The amount of terrorist activity in Europe at the moment is very very low, certainly compared to the 1970s. The only candidate really might be dissident IRA members, but that of course is pure speculation.

Do you think there might be a cover-up about what has happened?

No, we'll find out. The problem is that the building is very obvious, everybody knows it, an explosion has happened, and it's very difficult to hide it. There will have to be an inquiry. Questions are bound to be asked of the foreign secretary. I think we will find out.

Stephen Dorril is the author of With Friends Like These - MI6 - Inside the Covert World of Her Majesty's Secret Intelligence Service.